It has been demonstrated in this laboratory that lymphocyte transformation in vitro is both a sensitive and specific indicator of contact hypersensitivity. This has been demonstrated in the guinea pig with two haptens: dinitrofluorobenzene and paraphenylenediamine. We now propose to employ this technique in the study of contact allergens commonly encountered in industry: chromium, nickel, epoxy resin constituents, etc. With the information obtained from our work on the cellular responses in contact hypersensitivity, we plan to carry out additional studies with both immune cells and circulating antibody to evaluate the relative roles of these immune components in both iatrogenic and spontaneous ("hardening") desnesitization of contact hypersensitivity.